In today's wireless communication networks, user devices such as electronic book readers, cellular telephones, personal digital assistants (PDAs), portable media players, tablet computers, and netbooks use technologies like Global System for Mobile Communications (GSM or 2G), Universal Mobile Telecommunications System (UMTS or 3G), or more recent 3GPP Long Term Evaluation (LTE or 4G) for wireless communication. These user devices typically have to search for cells rather frequently.
A recent introduction to the area of wireless communication networks is the femtocell. A femtocell is a small wireless access point that transmits voice and data cellular signals. A femtocell is typically designed for use in a home or a small business. It connects to a mobile operator's network through the broadband Internet connection of the end user of the femtocell. Current femtocell designs typically support a limited number of user equipment (user device), for instance up to 16 mobile phones in an enterprise setting.
A closed subscriber group (CSG) is a 3GPP term used to describe a limited set of users with connectivity access to a femtocell. When a femtocell is configured in CSG mode, only those users included in the femtocell's access control list are allowed to use the femtocell resources. On the other hand, a femtocell can be also configured in Open Access mode, in which any user is allowed access to the femtocell. When comparing non-CSG cells and CSG cells, it is the user device's priority to select and access the CSG cell because membership in a CSG cell is more cost-efficient and, in most cases, results in better service for the user device. On the network operator side, a user device's use of CSG cells offloads demand on the network's cells and thus is also preferred by the network as well.
A CSG cell provided by a femtocell is identified by a unique numeric identifier called CSG Identity or CSG ID, which is broadcast in system information by the femtocell hosting the CSG cell. To facilitate access control, a user device with one or more CSG cell subscriptions typically stores a CSG whitelist in a SIM card of the user device. The CSG whitelist contains one or more CSG IDs associated with the CSG cells on which the user device is allowed access.
The user device uses the CSG whitelist along with any CSG IDs broadcast by CSG cells in the user device's range for CSG cell selection and reselection operations. If a CSG cell is added to the system information of macro cells that surround the CSG cell, this may result in all UEs in or near the macro cell measuring the CSG cell. Such measurement would occur even if the user device is not a member of the CSG cells or if the user device does not subscribe to CSG cells at all. These unnecessary measurements would result in battery/current drain in the user device.